Up until now, in structures formed using a plurality of steel sheet members, resistance spot welding has been used to join superposed steel sheet members in accordance with the function and usage environment. Forming spot welds having nuggets so as to form a lap welded member comprised of a plurality of steel sheet members has been a common practice. For example, in an automobile body having a monocoque structure, to improve impact safety and fuel efficiency, the general practice has been to superpose high strength steel sheets and join the flanges (superposed parts) by resistance spot welding.
In recent years, in the assembly of car bodies, remote laser welding has come to be used instead of the conventionally used resistance spot welding. Remote laser welding drives a galvanomirror to enable freedom of welding within a predetermined three-dimensional area. With remote laser welding, the galvanomirror attached to the front end of a robot arm can be moved quickly between welding points, so the welding tact time can be greatly shortened. Further, with remote laser welding, there is no shunting of welding current to existing welds such as with resistance spot welding, so it is possible to shorten the pitch between welds. The rigidity of the car body is also improved due to multipoint welding.
Remote laser welding is used in place of resistance spot welding, so usually a substantially circular laser weld, that is, a circular shape, oval shape, circular ring shape, oval ring shape, C-shape, long C-shape, multiple circular ring shape, or other welding shape, is used.
On the other hand, as the material forming a car body, in recent years, high strength steel sheet has been increasingly used. Cold pressed parts and hot pressed parts (hot stamped parts) of tensile strength 1500 MPa or other 1180 MPa or higher class high strength steel sheets have also come into use. For this reason, in the future, application of remote laser welding to 1180 MPa or higher class high strength steel sheet will be sought.
However, such high strength steel sheet has a metal structure mainly comprised of hardened martensite, so the surroundings of a laser weld are tempered, tempered martensite is formed, and the hardness falls. For example, in the case of 1500 MPa class high strength steel sheet, the hardness of the matrix material is Hv460 or so, but the hardness of the heat affected zone around a laser weld (hereinafter called “HAZ”) locally falls to Hv300 or so and that part softens. This locally softened part is called a “HAZ softened part”.
Such a HAZ softened part sometimes becomes the starting point of break of a steel sheet forming the car body at the time of impact of the vehicle. For example, in the case of a center pillar obtained by laser welding 1500 MPa class steel sheet to a flange as a reinforcement member, in a side collision test, sometimes a crack starts from the HAZ softened part of the flange and the center pillar breaks.
The phenomenon of breakage starting from the HAZ softened part formed by this laser welding remarkably occurs in a high strength steel sheet with a tensile strength of over 1180 MPa. In particular, it is remarkable in a laser weld of a tensile strength 1180 MPa or more high strength steel sheet formed with a hardened structure by a continuous annealing facility having a water cooling function or a high strength steel sheet formed by hot stamping (hot pressing). Remote laser welding enables the pitch between welds to be shortened so forms a large number of welds at a flange. However, if forming a large number of laser welds, a large number of HAZ softened parts are also formed, so that risk of break rises.
No countermeasures against break due to a HAZ softened part formed in a high strength steel sheet have been reported up to now. However, it has been reported that a similar phenomenon also occurs in the case of resistance spot welding.
NPLT 1 discloses treating a part in an A-pillar formed by hot stamping where there is a risk of break upon impact by heat treating it at the time of hot stamping so as to lower the strength of the matrix material and thereby inhibit HAZ softening resulting from resistance spot welding. Due to this, it is possible to prevent break of a structural member starting from the HAZ softened part.
NPLT 2 discloses the method of tempering the flange part of a B-pillar formed by hot stamping by high frequency heating so as to lower the strength of the matrix material so that HAZ softening does not occur even if performing resistance spot welding and thereby preventing break of the structural member starting from the HAZ softened part.
Further, as similar art, for example, PLT 1 discloses the art of combining laser welding and spot welding to ease the stress of a weld and suppress delayed break when welding high strength steel sheets to form a structural member for an automobile. PLT 2 discloses the art of laser welding a metal material to form a continuous weld along spot welds as art for improving the joint by welds. Furthermore, PLT 3 discloses the art of laser welding a spot weld or the surroundings of a spot weld as art for improving the joint strength of a weld.